We demonstrate that frontside scattering structures combining a metal nanomesh transparent electrode with dielectric nanosphere (NS) arrays may improve the performance of ultrathin crystalline silicon (c-Si) solar cells. The increased light scattering as characterized by increased haze from these structures leads to longer path lengths within the c-Si and thus, higher short-circuit current densities and improved power conversion efficiencies. We demonstrate a 69% improvement in power conversion efficiency with metal nanomesh/NS coatings compared to indium tin oxide. Furthermore, we demonstrate the ultrathin film c-Si solar cells are robust under repeated bending.

This guest editorial introduces the special section on TADF OLEDs.

The photophysical properties of six types of carbazole benzonitrile (CzBN) derivatives are investigated in different solvents to examine the thermally activated delayed fluorescence (TADF) activation via reducing the energy gap between the singlet charge-transfer and triplet locally excited states, ΔEST(LE). Relative to the ΔEST(LE) values for the CzBN derivatives in the low polarity solvent toluene (ϵ∼2), a reduction of ΔEST(LE) for the CzBN derivatives in the polar solvent acetonitrile (ϵ∼37) was confirmed while maintaining fairly constant ΔEST values. Notably, TADF activation was observed in acetonitrile for some CzBN derivatives that are TADF inactive in toluene. A numerical analysis of various rate constants revealed the cause of TADF activation as an increase in the reverse intersystem crossing rate and a suppression of the non-radiative decay rate of the triplet states. The positive effect of ΔEST(LE) was limited, however, as an excessive decrease in ΔEST(LE) facilitates the nonradiative deactivation of the triplet states, leading to a loss of the TADF efficiency. This paper shows that ΔEST(LE) provides a measure of TADF activation and that appropriate regulation of ΔEST(LE) is required to achieve high TADF efficiency.

Long-wavelength chromophores are developed based on fluorescein derivatives by introducing a strong acceptor 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile (TFM). By changing the number of TFM, we obtained two chromophores, which are named DTFM-DCF and STFM-DCF, respectively. Due to the large, rigid π-conjugated structure and strong electron-withdrawing capability of TFM, the maximum emission wavelengths of these molecules span from 681 to 755 nm in dichloromethane. Meanwhile, these two chromophores, which exhibit the property of thermally activated delayed fluorescence, have long-lived luminescence with 9.02  μs for STFM-DCF and 0.43  μs for DTFM-DCF in deaerated dichloromethane at room temperature.

A four-level model consisting of a higher triplet excited state (T2), the lowest singlet and triplet excited states (S1 and T1), and the ground state was previously used to understand emission properties of a thermally activated delayed fluorescence (TADF) emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). In this report, we discuss the four-level model in more detail and apply to the other two TADF emitters, i.e., 1,2,4,5-tetrakis(carbazol-9-yl)-3,6-dicyanobenzene (4CzTPN) and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN), in order to determine their excited-state structures. It is suggested that in all the emitters T2 lies between S1 and T1 and play an essential role in the emitting process. In 4CzTPN, phosphorescence from T2 is clearly observed around 100 K as in 4CzIPN. Compared to the other two emitters, 2CzPN has a wider energy gap between S1 and T1 so that delayed fluorescence at room temperature is thought to be mixed with phosphorescence. Because of this mixing, the spectrum characteristic of phosphorescence from T2 in 2CzPN cannot be identified.

Quantitative optical analyses were conducted on the mechanisms of impressively high electroluminescence (EL) efficiency (external quantum efficiency of up to 37%) achieved in previously reported blue organic light-emitting devices (OLEDs) using thermally activated delayed fluorescence emitters based on acridine–triazine hybrids. In addition to high photoluminescence quantum yields and preferentially horizontal emitting dipoles, optical simulation shows that the use of both low-index hole-transport layers (HTLs) and electron-transport layers (ETLs) also substantially contribute to enhanced optical outcoupling efficiencies and EL efficiencies of these devices. Further analyses on optical mode distributions and partitions in devices reveal significantly different optical outcoupling enhancement mechanisms for adopting low-index HTLs (i.e., reduced overall waveguided modes and enhanced microcavity effect) or adopting low-index ETLs (i.e., reduced surface plasmon and transverse magnetic waveguided modes), and their effects are combined to give even larger enhancement when reducing refractive indexes of both. Results of this work clearly indicate that optical properties of carrier-transport layers, in addition to their electrical properties, are critical factors and should also be carefully considered for future development of high-efficiency OLEDs.

We report a molecular design approach for blue-emitting thermally activated delayed fluorescence (TADF) molecules. The two TADF emitters, (4-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)phenyl)(phenyl)methanone (3PyCzBP) and (4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl)(phenyl)methanone (4PyCzBP), possess a pyridine-functionalized carbazole donor and a benzophenone acceptor. Both compounds show broad charge-transfer emission in dichloromethane with a λmax at 497 nm and a photoluminescence quantum yield, ϕPL, of 56% for 3PyCzBP and a λmax at 477 nm and a ϕPL of 52% for 4PyCzBP. The ϕPL decreased to 18% and 10%, respectively, for 3PyCzBP and 4PyCzBP in the presence of O2 confirming that triplet states involved in emission. The poly(methyl methacrylate) (PMMA)-doped (10 wt. %) films show blueshifted emission with λmax at 450 and 449 nm for 3PyCzBP and 4PyCzBP, respectively. The maximum ϕPL of 23.4% is achieved for these compounds in PMMA-doped film. The difference in energy between the singlet and triplet excited states (ΔEST) is very small at 0.06 and 0.07 eV for 3PyCzBP and 4PyCzBP, respectively. Multilayer organic light-emitting diode devices fabricated using these molecules as emitters show that the maximum efficiency (EQEmax) of the blue devices is 5.0%.

A butterfly shaped orange–red thermally activated delayed fluorescent (TADF) emitter, named ACFO, is developed by integrating electron-donating (D) 9,9-dimethyl-9,10-dihydroacridine units into γ-positions of an electron-accepting (A) fluorenone core to form a D-A-D configuration. ACFO emits intense orange–red light peaking at 600 nm in toluene solution, which is significantly red-shifted compared with its benzophenone-based counterpart. Imparted with a small singlet–triplet splitting energy of 0.13 eV, ACFO exhibits distinct TADF feature with a short delayed fluorescent lifetime of 3.8  μs and displays a moderate photoluminescence quantum yield of 31%. Consequently, employing ACFO as an emitter results in orange–red organic light-emitting diodes that exhibit a maximum external quantum efficiency of nearly 11%, corresponding to a nearly 100% exciton utilization efficiency in electroluminescence process. Our finding demonstrates for the first time that fluorenone derivatives can be employed as TADF emitters for high-efficiency fluorescent organic light-emitting diodes.

Pyrimidine is an electron-deficient azaaromatic compound containing two nitrogen atoms at 1, 3-positions that plays a key role as an organic semiconductor or semiconducting material. Because of the high electron-accepting property induced by C═N double bonds and due to its coordination ability, pyrimidine has been incorporated as a building block in phosphorescent emitters, fluorescent emitters, bipolar host materials, and electron transporting materials in organic light-emitting devices (OLEDs). Recently, pyrimidine-based thermally activated delayed fluorescent emitters combined with various electron donors have been developed, and their device performances were far better than those based on conventional fluorescent emitters. In this review, recent progress of pyrimidine-based OLED materials is presented and accompanied by a historical overview, current status, key issues, and outlook for the next generation of high-performance OLED materials.

The early history of thermally activated delayed fluorescence (TADF) studies is summarized prior to the popularization of TADF-based materials by Adachi and his group for organic light-emitting diodes (OLEDs) in the third-generation of OLEDs. Then, some insights into reverse and forward intersystem crossing (RISC and ISC) in TADF are argued based on earlier experimental results.

We fabricated well-ordered, crystalline mono- and multilayers of disk-shaped thermally activated delayed fluorescence (TADF) molecules such as 4CzIPN, 2CzPN, and 5CzBN. The slow deposition of these molecules on flat substrates such as Ag(111) at room temperature resulted in the formation of well-ordered and homogeneous monolayers. Moreover, the multilayer of the 4CzIPN was also found to be well-ordered and flat when deposited on highly oriented pyrolytic graphite (HOPG). The electronic states of the crystalline monolayer and multilayer of 4CzIPN were found to be nearly the same, suggesting that the electronic states of both layers are not altered significantly by adsorption on substrates. Indeed, we also confirmed the delayed fluorescence from the crystalline multilayer of 4CzIPN on the HOPG substrate even in an ambient condition. These results show promising applications of crystalline films of disk-shaped TADF-molecules such as 4CzIPN for organic light-emitting diodes devices with high outcoupling efficiency.

This guest editorial introduces and summarizes the Special Section on the Festschrift of the 70th Birthday of Zeev “Valy” Vardeny

We describe different types of photonic structures that allow tunability of the photonic band gap upon the application of external stimuli, as the electric or magnetic field. We review and compare two porous one-dimensional (1-D) photonic crystals: in the first one, a liquid crystal has been infiltrated in the pores of the nanoparticle network, whereas in the second one, the optical response to the electric field of metallic nanoparticles has been exploited. Then, we present a 1-D photonic crystal made with indium tin oxide (ITO) nanoparticles, and we propose this system for electro-optic tuning. Finally, we describe a microcavity with a defect mode that is tuned in the near-infrared by the magnetic field, envisaging a contact-less magneto-optic switch. These optical switches can find applications in information and communications technologies and electrochromic windows.

The third- and fifth-order nonlinear optical components are extracted using z-scan under a series of irradiances for some common reference materials rhodamine B(RhB) and carbon disulfide (CS2) as well as D-A fluorophores triphenylamine-phenylaldehyde (TPA-PA) and 2diphenylamine-tetraphenylethylene-phenylaldehyde (2DPA-TPE-PA). By differentiating the third- order optical nonlinearities from high-order ones, the two-photon absorption (2PA) cross sections of RhB, TPA-PA, and 2DPA-TPE-PA estimated from z-scan are almost all consistent with those estimated from two-photon excited fluorescence, whereas the second-order refractive indices of CS2 and TPA-PA in THF solution agree with reported values in publications as well. If the z-scan curve is captured at only a single irradiance, the 2PA cross sections will probably be overestimated. As for the fifth-order optical nonlinearities, the excited-state absorption cross sections of RhB are reduced with an increase in its concentration in methanol solutions. A more comprehensive photophysical picture of the nonlinear absorption and refraction processes within these samples can be drawn from their z-scan results.

We used a variety of optical spectroscopies to investigate the charge excitations and correlated infrared (IR)-active and Raman-active vibrations in poly[(difluoro-benzothiadiazol-diyl)-alt-(di(2-octyldodecyl)-quaterthiophen-diyl)], PffBT4T, a π-conjugated donor–acceptor (DA) copolymer, which, when blended with fullerene PCBM molecules, serves as an active layer in high-performance photovoltaic solar cells. The applied optical spectroscopies in films of pristine PffBT4T and PffBT4T/PCBM blend include absorption, photoluminescence, electroabsorption, photoinduced absorption (PA), and resonant Raman scattering. We found that the PffBT4T copolymer chain contains 11 strongly coupled Raman-active vibrational modes, which are renormalized upon photogeneration of charge polarons onto the chain. As the lower energy polaron absorption band overlaps with the renormalized vibrational modes, they appear in the PA spectrum as antiresonance (AR) lines superposed onto the induced polaron absorption band. We show that the Raman scattering, doping induced, and photoinduced AR spectra in PffBT4T are well explained by the amplitude mode model (AMM), where a single vibrational propagator describes the renormalized Raman modes and their related photoinduced AR intensities in detail. Surprisingly, we found that two of the IR-active modes in the pristine copolymer must be included in the AMM propagator for explaining the complete photoinduced AR spectrum. This feature is unique to DA-copolymers and indicates that some intrachain C_2v symmetry breaking occurs because of the different electron affinities of the donor and acceptor moieties.

The effects of molecular blending on the charge transport properties of organic semiconductors are studied based on ab initio DFT calculation and quantum nuclear tunneling model coupled with kinetic Monte-Carlo simulation. Two model compounds, [(2,3,9,10-tetrachloropentacene-6,13-diyl)bis(ethyne-2,1-diyl)]bis(triisopropylsilane) (4Cl-TIPS-P) and [(1,2,3,4,8,9,10,11-octafluoropentacene-6,13-diyl)bis(ethyne-2,1-diyl)]bis(triisopropylsilane) (8F-TIPS-P), are blended homogeneously with different ratios, and the system is studied at a temperature ranging from 50 to 300 K. Our result shows that, at high temperature, blending leads to a smooth shift in mobility, whereas at a low temperature, one of the components tends to become traps, thus hamper the charge transport at low concentration. It is found that at 50 K, blending 0.995 mol. % of 4Cl-TIPS-P with 0.005 mol. % of 8F-TIPS-P, whose site energy is 48.1 meV lower than that of 4Cl-TIPS-P, would reduce the electron mobility by three orders of magnitude from that of pristine 4Cl-TIPS-P due to the trapping effect. The results are in agreement with the experimental observations.

Organometallic hybrid perovskites have attracted intense attentions recently, as a new family of solution processable semiconductors for photoenergy conversion and light emission purposes. Specifically, CH₃NH₃PbBr₃ is one type of emissive perovskites, but its quantum efficiency largely depends on the preparation procedure. Here, we use the fluorescence lifetime imaging (FLIM) technique to investigate the relation between microscopic structures and photoluminescence (PL) in CH₃NH₃PbBr₃ polycrystalline films. By dripping poor solvents (chlorobenzene or chloroform) to accelerate the crystallization during the film preparation, we could decrease the crystal domain size from 10  μm down to 500 nm, and the corresponding PL intensity increases significantly. From the FLIM characterization of these films, we find that the PL emission is mostly from the edge of crystal domains. The PL dynamics indicates that the radiative decay of edge state is much more efficient than that of the bulk state, and the bulk state of photoexcitation undergoes an energy transfer to the edge state. This finding explains the origin of enhanced PL from CH₃NH₃PbBr₃ films when treated with poor solvents and provides useful information for further improvement on the PL efficiency of hybrid perovskite materials.

Blended films of poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV)/[6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) were annealed at 70°C under rotational magnetic field (RMF) (∼350  G with 600 RPM) for 10 min. At low temperatures, the films prepared with RMF contained new spectral features. The photoluminescence (PL) spectra of films prepared under RMF had two peaks (1.59 and 1.73 eV), whereas films prepared without RMF had a single PL band at 1.75 eV. Furthermore, the photoinduced absorption (PIA) spectra of films prepared with RMF showed an additional PIA band at 1.15 eV, along with the well-known PIA band at 1.35 eV due to excited state absorption by polarons. We conclude that the spectral features in MEH-PPV/PCBM films were due to charge transfer complexes (CTC), the formation of which is promoted by RMF. Our results suggested that applying proper magnetic field while preparing polymer/fullerene film could improve the formation of CTC. We discuss the relevance to solar cell applications.

We have fabricated a spin photovoltaic device composed of a vertical spin-valve based on hybrid organic–inorganic perovskite as a spacer layer, of which resistance may be tuned by external light illumination. The magnetoconductivity of this device may be tuned from zero to reach exceptionally high values of 100 k% by controlling the illumination intensity close to the open circuit voltage. In addition, the device photocurrent can be also turned by sweeping the external magnetic field when the effects of the light intensity and applied bias voltage are judicially balanced.

We use drift-diffusion simulations to clarify the effect of a steady-state depletion layer on the mobility determination using the charge extraction by linearly increasing voltage (CELIV) technique in case of extraction of doping-induced charge carriers. Accounting for the steady-state depletion layer gives rise to a saturation of the extraction current transients to a capacitive regime limited by the depletion layer capacitance. As a consequence of this saturation effect, the standard mobility formulas can overestimate the mobility by several orders of magnitudes if the voltage ramp-up rate is not sufficiently large. Based on the simulations, we present an expression to extract the mobility from CELIV, which takes the displacement current of the depletion layer into account.

Polymer nonfullerene solar cells are emerging as an alternative of polymer fullerene solar cells. However, maximizing the short-circuit density and open-circuit voltage is a critical issue in these solar cells. Here, using ultrafast spectroscopy, we measured exciton relaxation and charge separation dynamics in polymer nonfullerene blend with low driving force. Our study indicates high polaron yield despite the energy loss of as low as 0.59 eV. This suggests that the small driving force has minimum detrimental effect in realizing high performance in polymer nonfullerene solar cells.

Titanium dioxide (TiO₂) pastes containing secondary particle with varying sizes are prepared by modulating the dispersion time of the dispersion liquid. The appearances, mesoscopic structures, and transmission spectra of the TiO₂ layers constructed using these pastes depend on the sizes of the secondary particles. However, the dye-sensitized solar cells (DSSCs) constructed using different pastes show similar photoelectric conversion performances despite the different mesoscopic structures and transmission properties of the TiO₂ layers. The DSSCs based on the experimental TiO₂ pastes show lower photoelectric conversion efficiencies compared with the DSSC constructed using a commercially available reference paste. These results suggest that the primary TiO₂ particles have a greater influence on the photoelectric conversion performance of the DSSC than the secondary particles. The incident photon-to-current conversion efficiency spectra of the DSSCs based on the TiO₂ pastes exhibit a slight dependence on wavelength. Thus, it is possible that minor light-scattering effects resulting from the mesoscopic structure of TiO₂ occur in the DSSCs constructed using the pastes. These results indicate that TiO₂ pastes with fixed primary particle sizes but different secondary particle sizes can be used to change the appearance or design of DSSCs without affecting their photoelectric conversion performance.

We demonstrate index-tunable metamaterials working in the terahertz (THz) frequency range based on double-layered closed-ring resonator (CRR) arrays. The double-layered CRR arrays have a narrow-band transmission peak in a relatively wide stop band, and that peak shows spectral shift by slightly shifting relative position of the arrays or changing dielectric constant of the dielectric media inserted between CRR arrays. We show by numerical simulations that the effective refractive index can be widely tuned from un-naturally high positive to near-zero and negative values. Our approach may be utilized to develop THz active devices.

We have investigated spin transport in films of polyfluorene, a π-conjugated polymer, using the technique of “spin pumping” from a ferromagnet substrate, namely Ni₈₀Fe₂₀ in a NiFe/polyfluorene/Pt trilayer device at room temperature. Pure spin current (without carrier injection) is generated in the polymer film by microwave excitation of the NiFe magnetic moment at ferromagnetic resonance conditions. The induced spin current through the ferromagnet/polymer interface crosses the polymer layer and is detected by a Pt overlayer in the device, where it is converted into electric current via the inverse spin Hall effect. We have successfully determined the spin diffusion length, λ_S, in the polyfluorene film by varying the polymer thickness in the trilayer structure, and found λ_S  =  118  ±  9  nm. We also measured the charge-carrier mobility, μ in polyfluorene film using the time-of-flight technique and found it to be affected by dispersive transport. From the obtained λ_S and μ values, we estimated the spin relaxation time in polyfluorene to be ∼5  μs at room temperature.

Organic spintronics has been an active research field since the demonstration of large magnetoresistance in a thick organic spin valve, in which carrier spins are injected into the organic film sandwiched between two ferromagnetic electrodes. In such spin-injection devices, spin relaxation time and spin diffusion length are key properties that control spin-dependent transport and dictate the device design. Here, we compare spin relaxation and diffusion behaviors in organic solids due to spin-dependent interactions including spin-orbit coupling (SOC), hyperfine interaction (HFI), and exchange. It is found that for SOC-induced spin relaxation, the spin diffusion length is essentially determined by the spin admixture parameter and insensitive to magnetic field and carrier hopping rate, whereas for HFI-induced spin relaxation, it increases rapidly with both the magnetic field and hopping rate. In devices with high-density carriers, where exchange-induced spin motion dominates over carrier hopping, the spin diffusion length is limited by SOC.

Ambipolar carrier mobility in a thin film of hexyl-substituted phthalocyanine (C6PcH₂), which is a promising donor material for solution-processed organic thin-film solar cells, has been studied by measuring photogenerated charge carriers extracted under a linearly increasing voltage. As the extraction transient current of the C6PcH₂ thin film could not be observed in a conventional device structure composed of a single organic layer, an evaporated layer of C₆₀ or pentacene was introduced as a charge-separation layer to determine the hole or electron mobility, respectively. An extraction transient current clearly appeared owing to the efficient photo-induced charge generation at the interface between the C6PcH₂ and charge-separation layers. The hole and electron mobilities were evaluated by a proposed analytical model, which took into account the carrier generation at the interface and the carrier diffusion during the delay time.

Long-range corrected time-dependent density functional theory has been used to study the solvent effect on excited state properties of PCPDTBT:PCBM (Poly[2,6-(4,4-dimethyl-4H-cyclopenta[2,1-b:3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]:[6,6]-Phenyl-C61-butyric acid methyl ester) molecular system. A polarizable continuum model has been applied within the linear response (LR) and state-specific (SS) approaches to account for the dielectric environment. The results show that the influence of the solvent depends on the nature of the excitations. For neutral excitonic states that are essentially localized on a single molecule, the solvent has little or no effects on the excitation energies according to both solvent schemes. On the other hand, for states with a significant amount of charge transfer (CT), the SS approach predicts a sufficient decline in the excitation energy as the dielectric constant increases so that the CT state can be stabilized to the lowest excited state, whereas the LR counterpart shows almost no change. The comparison of two solvent approaches is discussed.

We investigate the magnetic field effects in thin-film diodes made of the conducting polymer poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) as a function of temperature and electrical current. Magnetoresistance of these devices can be measured to high precision on two distinct magnetic field scales: <3  mT, where a pronounced nonmonotonic magnetoresistance response can be resolved, owing to weak hyperfine coupling, and at intermediate magnetic fields, ranging between 3 and 10 mT, where strong monotonic magnetoresistance is seen. The detailed examination of the magnetoresistance effects in both regimes allows one to scrutinize the accuracy of the underlying models for the behavior of these kinds of materials.

We report on the photophysical properties of a soluble thienylene-vinylene π-conjugated polymer, namely imide poly-thienylene vinylene. Ultrafast pump probe spectroscopy reveals that a broad photoinduced absorption (PA) band is immediately photogenerated along with a narrower PA from the photoinduced singlet excitons. The broad PA is susceptible to magnetic field and thus is assigned to a correlated triple pair state. This feature shows a long lifetime persisting into the μs time domain, in contrast to the singlet exciton PA, which quickly decays on a 10-ps timescale. The steady-state PA spectrum is identical to the transient PA spectrum of the triplet pair state but shows a magnetic field response that is typical to isolated triplet excitons.

We have fabricated microdisk lasers of colloidal quantum dots in circular and elliptical forms with different aspect ratios. By characterizing the laser emission spectrum under optical pumping and through mode simulation calculations, we demonstrate that the elliptical resonators can display two sets of whispering-gallery modes that interact to varying degrees depending on the aspect ratio. This causes significant mode splitting in the emission spectra when the mode interaction is at maximum. We also performed angular-dependent laser emission measurements for characterizing the emission pattern of the microlasers. We found that the emission pattern becomes less isotropic and more directional as the boundary deviates further from circular symmetry. In addition, we demonstrate that the emission directional properties of these microlasers can be further tailored by coupling pairs of elliptical microcavities together in different manners, where the long or short elliptical axis interacts most strongly.

The conjugated polymer MEH-PPV consists of two basic types of chromophores that we characterize as “isolated” and “aggregated.” Because the aggregated chromophores have lower emissive yields but are favored by energy migration, we show that preferential bleaching of the aggregated chromophores can result in substantially improved fluorescence quantum yield. We study a wide variety of sample types including solutions, dilute films, neat films, and core-shell particles and show that the degree to which luminescence improvements are possible depends on the organization of the chromophores.

We present a comprehensive review on the optical properties in pristine low bandgap copolymers, namely PTB7 and PDTP–DFBT, which are used as electron donors in copolymer/phenyl C71 butyric acid methyl ester blends for high efficiency solar cell devices. The copolymer backbone chain comprises of donor (D) and acceptor (A) moieties, which lower the band gap to the near-IR spectral region. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in D–A copolymers we find at short times coexistence of two primary photoexcitation species, namely SE and triplet–triplet (TT) pairs, which are directly photogenerated upon photon absorption from the ground state within 300 fs, our transient time resolution. Using the transient magnetophotoinduced absorption (t-MPA) spectroscopy, we reveal the spin coupling between the SE and TT spin states from their correlated t-MPA responses. In addition, we show that the TT species dissociates into two individual triplet excitons (TE) in the picosecond time scale; however, the two-geminate TEs are entangled and maintain their spin coherence into the microsecond time domain.

This paper presents a low temperature, solution-based processing method of highly transparent, sparse networks of carbon nanotubes via annealing process that dramatically improves the conductivity of thin films of octadecylamine functionalized highly soluble single-wall carbon nanotubes by up to five orders of magnitude. This increase in conductivity obtained at low temperatures allows for the creation of transparent conducting carbon nanotube (CNT) films via printed deposition of contacts for photovoltaic, light emitting, and display devices. An increase in films conductivity has been shown with process temperatures of 200°C at normal atmospheric pressure. The dependence between the sheet resistance of CNT layers and the annealing parameters is analyzed together with Raman and FTIR data, suggesting a relationship between the loss of octadecylamine functional groups along with the healing of CNT defects during the annealing process and the dramatic conductivity improvement of CNT layers.

Hyperfine interaction (HFI) has been considered as a dominant spin mixing mechanism in conventional semiconducting polymers causing large magnetoconductance (MC) in organic diodes. However, the relationship between the MC width or HFI strength and the MC magnitude has not been investigated. We studied the correlation between the width and the magnitude of the MC response in organic diodes made by several conventional π-conjugated semiconducting polymers. First, by comparing the MC responses in electron- and hole-only unipolar devices made by the same polymer, we found that the electron-only device with a larger MC width always show a larger MC magnitude than that in the corresponding hole-only device. Second, we intentionally decreased and increased the charge localization or HFI strength in these unipolar devices by controlling their annealing temperature and UV irradiation, respectively. We found that the MC magnitude in these unipolar devices generally increases when the HFI strength increases but with different rates. We conclude that the width of MC or HFI strength is a crucial but not a unique factor that influences the MC magnitude. Finally, although the HFI in bipolar devices is smaller than that in the corresponding electron-only devices, the MC magnitude in bipolar devices is always larger than that in the electron-only devices suggesting that their underlying mechanisms are different.

Nanostructured black silicon (bSi) exhibits a broadband antireflection (AR) response due to graded-index and scattering effects, unlike traditional quarter-wavelength dielectric AR coatings. We present various techniques to improve the front- and back-surface performance of nanostructured bSi solar cells. Ammonium dihydrogen phosphate (ADP) is used for proximity doping to reduce the physical impact on the bSi nanostructures during front-surface emitter formation. An optimum concentration of 2 wt. % of ADP is found to result in a typical solar cell emitter sheet resistivity of 50  Ω  /  sq. Potassium hydroxide is used to etch off the highly doped region of the bSi solar cell front emitter, which results in lower surface recombination and up to a 23% increase in short wavelength (400 to 600 nm) internal quantum efficiency of the bSi solar cell. To reduce the series resistance and enhance surface passivation, forming gas anneal is employed, improving bSi cell’s overall efficiency by over 31%. By optimizing the back-surface-field formed by sputtered aluminum (Al), the backside recombination rate is reduced, improving external quantum efficiency by up to 11% in the long wavelength (>900  nm) region.

A two-dimensional model was developed to simulate the optoelectronic characteristics of indium-gallium-nitride (InξGa1  −  ξN), thin-film, Schottky-barrier solar cells. The solar cells comprise a window designed to reduce the reflection of incident light, Schottky-barrier and ohmic front electrodes, an n-doped InξGa1  −  ξN wafer, and a metallic periodically corrugated backreflector. The ratio of indium to gallium in the wafer varies periodically throughout the thickness of absorbing layer of the solar cell. Thus, the resulting InξGa1  −  ξN wafer’s optical and electrical properties are made to vary periodically. This material nonhomogeneity could be physically achieved by varying the fractional composition of indium and gallium during deposition. Empirical models for indium nitride and gallium nitride were combined using Vegard’s law to determine the optical and electrical constitutive properties of the alloy. The nonhomogeneity of the electrical properties of the InξGa1  −  ξN aids in the separation of the excited electron–hole pairs, whereas the periodicities of optical properties and the backreflector enable the incident light to couple to multiple guided wave modes. The profile of the resulting charge-carrier-generation rate when the solar cell is illuminated by the AM1.5G spectrum was calculated using the rigorous coupled-wave approach. The steady-state drift-diffusion equations were solved using COMSOL, which employs finite-volume methods, to calculate the current density as a function of the voltage. Midband Shockley–Read–Hall, Auger, and radiative recombination rates were taken to be the dominant methods of recombination. The model was used to study the effects of the solar-cell geometry and the shape of the periodic material nonhomogeneity on efficiency. The solar-cell efficiency was optimized using the differential evolution algorithm.

Solar panels are gaining global popularity for electrical energy generation. However, efficiency of static solar panels can vary during the day. Solar panels can have increased energy output if they are actively oriented to capture the most amount of sunlight as the sun moves in the sky. In order to achieve this active orientation, sun’s angular position needs to be estimated. Sun’s position in the sky and zenith angle can be determined by automatic analysis of images captured by a whole sky camera. As the purpose of a whole sky, sun tracking camera is to accurately identify solar zenith angle, a special camera integrated with a fisheye lens with equidistant (f-theta) mapping can be designed. Due to the angular linearity of equidistant fisheye lenses, solar zenith angle can be directly calculated after the position of the sun in a whole sky image is detected with digital image processing algorithms. This paper presents the optical design of a compact, high-resolution equidistant fisheye lens with low deviation from the f-theta distortion profile. The lens can be integrated with a commercial image sensor array to form a low-cost whole sky camera for accurate sun tracking in solar energy generation systems.

This paper reports photoluminescent studies of ATiO₃  :  Eu^2  +, Yb^2  + (A  =  Ca, Ba, Sr) perovskites synthesized by facile molten salt synthesis (MSS) technique. This synthesis technique uses fused salt NaCl to create an active reaction medium. The perovskites were synthesized at a sintering temperature of 950°C with the doping concentration of Eu^2  + and Yb^2  + ions varying from 0 to 2 mol. %. The study of crystallographic analysis set forth the orthorhombic, tetragonal, and cubic crystal structures of CaTiO₃  :  Eu^2  +, Yb^2  +; BaTiO₃  :  Eu^2  +, Yb^2  +; and SrTiO₃  :  Eu^2  +, Yb^2  + perovskites, respectively, with appreciable phase purity and homogeneity. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) micrographs show agglomerated globules with the average diameter varying from 180 to 360 nm. The UV–visible absorption spectra of synthesized phosphors show a number of peaks centered between 450 and 550 nm with increased absorbance count confirming the efficient energy transfer between Eu^2  + and Yb^2  + ions. The Fourier transform infrared (FTIR) spectral analysis shows peaks around 500 to 750  cm^−  1 corresponding to bridging stretching modes of Ti–O–Ti. The sharp peaks in the range of 990 to 1260  cm^−  1 were characteristics of asymmetrical stretching of Eu^2  + and Yb^2  + impurity ions. The active vibrational modes of A–O were obtained in range of 1460 to 1680  cm^−  1. The photoluminescence emission spectra of ATiO₃  :  Eu^2  +, Yb^2  + (A  =  Ca, Ba, Sr) perovskites (1:1 mol. %) phosphor exhibits emissions at 460, 480, 500, 530, 550, 560, 585, 600, and 645 nm, which are resultants of efficient energy transfer and crystal field splitting influenced hypersensitive transitions of Eu^2  + ions and Yb^2  + ions. The sensitization-based efficient energy transfer between Eu^2  + and Yb^2  + ions results in increased lumen output of the codoped perovskite phosphors. The analysis of chromaticity parameters confirms the color tunability of the synthesized perovskite phosphors.

We are reporting the optical study of two types of poly (3-thienylene vinylene) (PTV) derivatives with linear and bulk side chain configurations. The first one, poly(3-dodecylthienylenevinylene) with controlled regioregularity (PTV-CR) with an alkyl side group, showed a high degree of crystallinity, whereas the second one, imide-PTV with an electron deficient imide side group, is amorphous. Continuous wave-photoinduced absorption revealed different features of the long-lived triplet absorptions, with the geminately generated triplet–triplet pair in imide-PTV bearing more intrachain characteristics with fast recombination kinetics (<10 μs). On the other hand, the isolated triplet in PTV-CR generated by intersystem crossing had more interchain characteristics with slow recombination kinetics (ms). The decay of the triplet–triplet pair in imide-PTV is monomolecular even at a high pump intensity of 240 mW/cm², whereas the isolated triplets in PTV-CR recombine bimolecularly at a very low pump intensity of 10 mW/cm². Grazing incidence X-ray diffraction, doping induced absorption, and various dependences of the photoinduced absorption spectrum have corroborated the finding that reduced interchain interaction favors the formation of a correlated triplet–triplet pair, a precursor to two independent triplets formed via fission.